Activated peptides and conjugates

ABSTRACT

Novel activated peptides and conjugates thereof, useful in diagnostic assays and therapeutics, and processes for the preparation thereof are disclosed.

[0001] The present invention relates to activated peptides, conjugates and methods of preparation thereof, and their use in diagnostic assays and therapeutics.

[0002] Numerous assays have been developed for the detection and determination of proteins, e.g., antibodies in biological fluids. One class, the immunoassays, which has evolved into an invaluable tool in diagnostics, is based upon the principle of specific binding of antibodies to haptens and/or antigens. In a typical immunoassay, a test sample, containing an analyte of interest, and an antibody, to which it specifically binds, are incubated, and then washed to separate free and bound analytes. An enzyme-labeled antibody that recognizes the resulting complex is added, incubated, and washed, and finally substrate, an enzyme detection system, is added and the labeled complex is detected and determined.

[0003] Conjugates of peptides specific to antibodies have been used advantageously in immunoassays. Conjugates of peptide analogues of viral proteins, that is, segments of the proteins bearing the epitopic sequence, for example, of the human immunodeficiency viruses (HIV) and labeled enzymes linked through a maleiimide moiety have been described as being particularly advantageous in the detection and determination of antibodies to HIV. See U.S. Pat. No. 5,294,536 granted to Paul S. Palumbo on Mar. 15, 1994 ('536-patent). The conjugates of activated peptides, prepared by the processes described in the '536-patent are not homogenous, the activated peptides being derived from the terminal amino group and/or the internal amino and hydroxyl groups of the peptide analogue. Interaction of the internal amino and/or hydroxyl groups of the peptide analogue and the crosslinking agent, in addition to that of the terminal amino group, diminishes the effectiveness of the epitopic centers of the peptide analogue thereby reducing the sensitivity of the assay for the detection and determination of specific binding antibodies. For example, interaction of the amino group of the lysine subunit and/or the amino and the hydroxy subunits of the serine subunit of the epotopic segment of the peptide analog of the HIV virus and the crosslinker reduces the sensitivity of the immunoassay for the detection or determination of antibodies to HIV, the greater the interaction of the internal amino and/or hydroxyl group relative to the terminal amino group, the lower the sensitivity of the assay. The processes for the preparation of activated peptides 3 and conjugates 5 described in the aforementioned '536-patent involve condensation of an isocyanatomaleiimide 1

[0004] wherein X is a spacer group with a peptide having a terminal amino group of formula 2

RNH₂  2

[0005] wherein R is the remainder of the peptide to form an activated peptide 3

[0006] wherein R and X are as above, which is then condensed with a thiolated enzyme of formula 4

R₁SH  4

[0007] wherein R₁ is the remainder of an enzyme to form a conjugate of formula 5

[0008] wherein X, R, and R₁ are as defined herein. The critical step in the process, the reaction of the crosslinker 1 with a peptide 2, characterized by the presence of a terminal amino group and internal amino and/or hydroxyl groups, affords an activated peptide 3 derived from the terminal amino group, as well as activated peptides derived from the internal amino and/or hydroxyl groups of the peptide, and combinations thereof. It has now been found that by performing the reaction of the isocyanatomaleiimide 1 with a peptide 2 as a salt of a strong protonic acid, the activated peptide is formed substantially free of activated peptides derived from internal amino and/or hydroxyl groups, e.g., of serine or lysine, i.e., the reaction takes place almost exclusively at the terminal amino group of the peptide 2 to regiospecifically form an activated peptide 3 with the epitopic segment of the peptide 2 essentially intact.

[0009] It has now also been found that the integrity of the binding region of the activated peptide 3 is maintained in the conjugate 5 and that use of the essentially homogeneous conjugate 5 of the invention in an immunoassay results in a marked improvement of the sensitivity of the assay.

[0010] More particularly, the present invention relates to activated peptides of formula 3

[0011] wherein X is loweralkylene, an aromatic carbocyclic moiety or a saturated carbocyclic moiety and R is the remainder of a peptide having a terminal primary amino group and free internal hydroxyl and/or amino groups, useful for the preparation of conjugates 5

[0012] wherein R and X are as defined above and R₁ is the remainder of an enzyme having a free thiol group, useful for the detection and determination of antibodies in samples of interest related to the human immunodeficiency viruses.

[0013] Preferred activated peptides 3 are those wherein X is an aromatic moiety; most preferred are peptides wherein X is phenyl, and R is the remainder of a peptide analogue having a terminal amino group selected from the group consisting of

[0014] As used throughout the specification and appended claims, the term alkylene refers to a saturated straight or branched chain hydrocarbon having 1 to 10 carbon atoms. Examples of alkylene groups are methylene (—CH₂—), ethylene (—CH₂CH₂—), butylene

[0015] The term “alpha-amino acid” refers to a compound characterized by the presence of a carboxylic acid group and an alpha-amino group bound to the same carbon atom

[0016] Examples of amino acids are alanine, valine, leucine, isoleucine, proline (or hydroxyproline), phenylalanine, tryptophan, methionine, glycine, serine, threonine, cysteine, tyrosine, asparagine, glutamine, aspartic acid, glutamic acid, lysine, arginine, and histidine.

[0017] The term “peptide” refers to polymers of two or more amino acids linked covalently through the carboxyl group of an amino acid and the amino group of another with the elimination of water. Examples of peptides are those compiled herein in the Sequence Listing.

[0018] The term “protein” refers to a macromolecule of one or more chains of amino acids bound covalently through peptide bonds

[0019] Proteins include enzymes, antibodies, antigens, peptides and the like.

[0020] The term “hapten” refers to a low-molecular-weight compound that reacts specifically with an antibody, but does not stimulate antibody production unless complexed with a carrier protein.

[0021] The term “antigen” refers to a substance or entity (usually a protein) that induces the direct production of antibodies.

[0022] The term “spacer group” refers to a moiety bridging the isocyanato and maleiimides groups and linking the protein and thiolated enzyme moieties of a conjugate. Examples of spacer groups are alkylene (hereindefined), aromatic carbocyclics, such as phenyl or naphthyl optionally having an alkylene group for attachment to the maleiimide moiety and dimethylamino, methoxy, ethoxy, methyl, ethyl, sulfonamido, sulfonic acid substituents, and saturated carbocyclics such as cyclohexyl, cyclohexylalkyl, cyclopentyl, cyclopentylalkyl, cycloheptyl, cycloheptylalkyl.

[0023] The term “regiospecific” refers to a process in which one specific structural or positional isomer is formed to the essential exclusion of other possible isomers.

[0024] The term “remainder” as applied to peptides refers to the moiety bound to the terminal amino group thereof, for example, the terminal amino group of the terminal glycine moiety of the sequence of amino acids shown below:

[0025] The term “remainder” as applied to an enzyme having a free thiol group refers to the moiety bound to the thiol group thereof, for example, specific enzymes such as beta-D-galactosidase, peroxidase, glucose oxidase and alkaline phosphatase, in which a thiol group has been introduced.

[0026] The N-terminal activated peptides 3 of the present invention are prepared by contacting an isocyanatomaleiimide 1 with a peptide 2 as a salt of a strong protonic acid, having in the remainder free amino and/or hydroxyl groups capable of interaction with the isocyanato moiety of the crosslinker 1, in a suitable solvent, conditions under which the terminal amino group regioselectively reacts with the isocyanato moiety of the crosslinker 1, without substantial interaction of the internal amino and/or hydroxyl groups with the isocyanato moiety. Included among strong protonic acids are hydrohalic acids such as hydrobromic acid and hydrochloric acid. Also included among such acids are haloacetic acids, for example, trichloroacetic and trifluoroacetic acid. Haloacetic acids are preferred. Trifluoroacetic acid is most preferred.

[0027] A variety of peptides 2 containing a terminal amino moiety are commercially available, many as salts of trifluoroacetic acid. In the event the peptide 2 is not available as the trifluoroacetic acid salt, hydrohalic and haloacetic acid salts may be prepared by conventional methods.

[0028] Suitable solvents include dipolar aprotic solvents, for example, dimethylformamide, dimethylacetamide, hexamethylphosphoramide, or dimethylsulfoxide. Dimethylformamide is the preferred solvent.

[0029] The reaction of a terminal amino group of a peptide 2 with a crosslinker 1 is carried out at a temperature compatible with the stability of the reactants, generally as determined by the stability of the peptide 2. Typically, the reaction is performed at about 30° C.

[0030] The relative amounts of the peptide 2 and crosslinker 1 are not critical. With about a three-fold excess of crosslinker 1 to peptide 2, good yields of the activated peptide 3 are obtained.

[0031] The reaction of a peptide 2 and crosslinker 1 is worked up by conventional methods and the product is purified by known chromatographic techniques, for example, reverse phase high performance liquid chromatography.

[0032] The activated peptide 2 was analyzed by various spectral techniques, for example, mass spectrometry; and the regiospecifity of reaction of the crosslinker 1 with the N-terminal amino group of the peptide 2 was established by aminopeptidase and trypsin digestion in combination with mass spectral analyses. See, for example, K. Arar, et al., Tetrahedron Letters, 34, 8087 (1993), ref 15, and B. Keil in The Enzymes, P. D. Boyer, Editor, Vol. 111, Academic Press, New York, N.Y., 1971, Chapter 8, for a discussion of these methods of determining the site of reaction of a peptide having multiple amino and/or hydroxyl groups with a crosslinker such as an isocyanatomaleiimide 1.

[0033] The yields of the activated peptide 3 of the present invention prepared by the reaction of an isocyanatomaleiimide 1 with a peptide 2 are uniformly high, the crude yields by chromatographic techniques falling within the range of about 49 to about 91%, the isolable yields falling within the range of about 47 to about 60%.

[0034] The purity of the activated peptides 3, the condensation products of isocyanatomaleiimides 1 and N-terminal peptides 2, was established by reverse phase high performance liquid chromatography.

[0035] The conjugates 5 of the present invention are useful in enzyme-linked immunoassays (ELISA) for the detection and determination of proteins, e.g., antibodies, particularly antibodies to human immunodeficiency viruses in a ELISA module of the OPUS® system. See H. J. Crowley and M. A. Bandin in The Immunoassay Handbook, D. Wild, Edition, Stockton Press, New York, N.Y., 1994, page 197. In such an assay, e.g., an analyte of interest of a biological sample, e.g., an antibody, and the conjugate are incubated, applied to antibody capturing support, washed with a substrate, incubated and detected. In particular, the sample of interest containing an antibody to a human immunodeficiency virus and the conjugate of a peptide derived from a glycoprotein of a human immunodeficiency virus are incubated, applied to a matrix of a fusion protein, washed with methyl umbelliferyl phosphate, incubated, and the amount of conjugate bound to the peptide specific antibody is detected and determined by fluorimetry.

[0036] Enzymes that contain a thiol group such as β-D-galactosidase and those that do not such as peroxidase, glucose oxidase and alkaline phosphatase, into which a thiol may be introduced, can be employed in the conjugation with activated peptides.

[0037] The activated peptides 3 of the present invention are also useful as therapeutics for the treatment of disease. See K. Arar, ibid., page 1 and references cited thereon, as well as Gary A. Koppel, Bioconjugate Chemistry, 1, 13 (1990).

[0038] The peptide starting materials are available from commercial sources.

[0039] The invention will now be further described with respect to specific preferred embodiments by way of examples, it being understood that these are intended to be illustrative only and the invention is not limited to the materials, processes, etc., recited therein.

EXAMPLE 1

[0040] N-Terminal Activation of Peptide BC202 (Seq ID No: 2) with p-Maleiimidebenzene Isocyanate

[0041] Synthetic peptide BC202 (Seq ID No: 2), consisting of 36 amino acids with an internal disulfide loop is derived from HIV-2 genome and is a portion of the envelope protein gp36. An N-terminal arginine, internal lysine and serine, and a C-terminal serine are potential sites for activation with p-maleiimidebenzene isocyanate (PMBI). In this example, 7.1 mg of the triflororacetic acid salt of BC202 was dissolved in 1.42 ml of dry dimethylformamide. To the solution a 3-molar excess (1.09 mg) of p-maleiimidebenzene isocyanate in 0.109 ml of dimethylformamide was added. The reaction mixture was incubated for 30 mins at 30° C., incubated with 4.59 ml of deionized water for 5 mins at 30° C., and centrifuged. The precipitate was collected, and the mother liquor was purified by reverse phase high performance liquid chromatography with a linear gradient of 30% -35% acetonitrile in 0.06% trifluoroacetic over 30 mins. The flow rate was 4.7 ml/min and the separation monitored at 220 nm and 280 nm. The appropriate fractions were collected and evaporated. The residue was dried to yield the activated peptide as a powder; yield 94% by high performance liquid chromatography, 60% by isolation.

[0042] The activated peptide was analyzed by mass spectrometry, aminopeptidase digestion, trypsin digestion in combination with mass spectrometry, back titration with mercaptoethanol and reaction with 5,5′-dithiobis(2-nitrobenzoic acid) and conjugated to alkaline phosphatase for functional testing in an HIV-2 assay.

[0043] The reaction and separation conditions for the preparation of the activated peptide of the invention, prepared by processes substantially similar to those as described in the example, and the yields of the products so obtained are summarized in the Table. TABLE Activated Peptide Separation % Yield PMBI Conditions and by HPLC Peptide* (molar ratio) Eluent** (Isolation Yield) D-23-N (SEQ ID  3X Isocratic 30% B ND NO:7) D-24-N (SEQ ID  3X Isocratic 30% B 90 NO:3) N-23-N (SEQ ID  3X Isocratic 30% B 86% (50%) NO:4) BC80 (SEQ ID 10X Isocratic 30% B 89% (60%) NO:1) BC80 (SEQ ID  3X Isocratic 30% B 77% NO:1) BCH132 (SEQ ID  3X Gradient 15-30% B 47% NO:9) 30 min. BCH408 (SEQ ID  3X Gradient 30-40% B 91% NO:6) 30 min. BCH178 (SEQ ID 20X Isocratic 25% B 87% NO:8) BC202 (SEQ ID  3X Gradient 30-35% B 94% (60%) NO:2) 30 min. BCH87 (SEQ ID  3X Gradient 32-37% B 74% (49%) NO:5) 30 min.

[0044]

1 9 17 amino acids amino acid single linear 1 GLY CYS SER GLY LYS LEU ILE CYS THR THR ALA VAL PRO TRP ASN ALA 1 5 10 15 SER 36 amino acids amino acid single linear 2 ARG VAL THR ALA ILE GLU LYS TYR LEU GLN ASP GLN ALA ARG LEU ASN 1 5 10 15 SER TRP GLY CYS ALA PHE ARG GLN VAL CYS HIS THR THR VAL PRO TRP 20 25 30 VAL ASN ASP SER 35 24 amino acids amino acid single linear 3 ASP GLN ALA ARG LEU ASN SER TRP GLY CYS ALA PHE ARG GLN VAL CYS HIS 1 5 10 15 THR THR VAL PRO TRP VAL ASN 20 23 amino acids amino acid single linear 4 ASN GLN GLN ARG LEU ASN LEU TRP GLY CYS LYS GLY LYS LEU ILE CYS 1 5 10 15 TYR THR SER VAL LYS TRP ASN 20 35 amino acids amino acid single linear 5 ARG ILE LEU ALA VAL GLU ARG TYR LEU LYS ASP GLN GLN LEU LEU GLY 1 5 10 15 ILE TRP GLY CYS SER GLY LYS LEU ILE CYS THR THR ALA VAL PRO TRP 20 25 30 ASN ALA SER 35 39 amino acids amino acid single linear 6 LYS ILE LEU ALA VAL GLU ARG TYR LEU LYS ASP GLN GLN LEU LEU GLY 1 5 10 15 ILE TRP GLY CYS SER GLY LYS LEU ILE CYS THR THR ALA VAL PRO TRP 20 25 30 ASN ALA SER GLY LYS LEU ILE 35 23 amino acids amino acid single linear 7 ASP GLN GLN LEU LEU GLY ILE TRP GLY CYS SER GLY LYS LEU ILE CYS 1 5 10 15 THR THR ALA VAL PRO TRP ASN 20 27 amino acids amino acid single linear 8 ASN GLN GLN SER ARG TRP GLY LEU GLY SER PRO ASN CYS HIS GLY PRO 1 5 10 15 ASP TRP ALA SER PRO VAL CYS GLN ARG HIS SER 20 25 22 amino acids amino acid single linear 9 LYS ILE GLU PRO LEU GLY VAL ALA PRO THR LYS ALA LYS ARG ARG VAL 1 5 10 15 VAL GLN ARG GLU LYS ARG 20 

We claim:
 1. A compound of the formula

wherein X is loweralkylene, an aromatic carbocyclic group or a saturated carbocyclic group and R is the remainder of a peptide having a terminal amino group and characterized by the presence of internal amino and/or hydroxyl groups.
 2. A compound according to claim 1 wherein X is an aromatic carbocyclic group.
 3. The compound according to claim 2 wherein X is phenyl.
 4. The compound according to claim 1 wherein R is the remainder of a peptide having a terminal amino group of the formula


5. A compound according to claim 1 wherein R is the remainder of a peptide having a terminal amino group of the formula


6. A compound according to claim 1 wherein R is the remainder of a peptide having a terminal amino group of the formula


7. A compound according to claim 1 wherein R is the remainder of a peptide having a terminal amino group of the formula


8. A compound according to claim 1 wherein R is the remainder of a peptide having a terminal amino group of the formula


9. The compound according to claim 1 wherein R is the remainder of a peptide having a terminal amino group of the formula


10. The compound according to claim 1 wherein R is the remainder of a peptide having a terminal amino group of the formula


11. The compound according to claim 1 wherein R is the remainder of a peptide having a terminal amino group of the formula


12. The compound according to claim 1 wherein R is the remainder of a peptide having a terminal amino group of the formula


13. A compound of the formula

wherein X is loweralkylene, an aromatic carbocyclic group or a saturated carbocyclic group and R is the remainder of a peptide having a terminal amino group characterized by the presence of internal amino and/or hydroxyl groups, substantially free of compounds, in which at least one of the internal amino and/or hydroxyl groups is bound to the carbon atom of the amide carbonyl group.
 14. A compound according to claim 13 wherein X is an aromatic carbocyclic group.
 15. A compound according to claim 14 wherein X is phenyl.
 16. The compound according to claim 13 wherein R is the remainder of a peptide having a terminal amino group of the formula


17. The compound according to claim 13 wherein R is the remainder of a peptide having a terminal amino group of the formula


18. The compound according to claim 13 wherein R is the remainder of a peptide having a terminal amino group of the formula


19. The compound according to claim 13 wherein R is the remainder of a peptide having a terminal amino group of the formula


20. The compound according to claim 13 wherein R is the remainder of a peptide having a terminal amino group of the formula


21. The compound according to claim 13 wherein R is the remainder of a peptide having a terminal amino group of the formula


22. The compound according to claim 13 wherein R is the remainder of a peptide having a terminal amino group of the formula


23. The compound according to claim 13 wherein R is the remainder of a peptide having a terminal amino group of the formula


24. The compound according to claim 13 wherein R is the remainder of a peptide having a terminal amino group of the formula


25. A process for the preparation of a compound of the formula

wherein X is loweralkylene, an aromatic carbocyclic group or a saturated carbocyclic group and R is the remainder of a peptide having a terminal amino group and characterized by the presence of internal amino and/or hydroxyl groups, substantially free of compounds, in which at least one of the internal amino and/or hydroxyl groups is bound to the carbon atom of the amide carbonyl group, which comprises contacting a compound of the formula

wherein X is as defined herein with a compound of the formula RNH₂ wherein R is as defined herein as a salt of a strong protonic acid.
 26. A process according to claim 25 wherein the strong protonic acid is a hydrohalic acid.
 27. The process according to claim 26 wherein the hydrohalic acid is hydrochloric acid or hydrobromic acid.
 28. A process according to claim 25 wherein the strong protonic acid is a haloacetic acid.
 29. A process according to claim 28 wherein the haloacetic acid is trifluoroacetic acid or tribromoacetic acid.
 30. The process according to claim 28 wherein the haloacetic acid is trifluoroacetic acid.
 31. A process according to claim 25 wherein a solvent is employed.
 32. A process according to claim 31 wherein the solvent is a dipolar aprotic solvent.
 33. A process according to claim 32 wherein the dipolar aprotic solvent is dimethylformamide, dimethylacetamide, hexamethylphosphoramide, dimethylsulfoxide, or acetonitrile.
 34. The process according to claim 33 wherein the dipolar aprotic solvent is dimethylformamide.
 35. A process according to claim 25 wherein the process is performed at a reaction temperature of from about 20° C. to about 40° C.
 36. The process according to claim 3542 wherein the reaction temperature is about 30° C.
 37. A compound of the formula

wherein X is loweralkylene, an aromatic carbocyclic group or a saturated carbocyclic group and R is the remainder of a peptide having a terminal amino group and characterized by the presence of internal amino and/or hydroxyl groups; and R₁ is the remainder of a compound characterized by the presence of a thiol group bound thereby to the 3-position of the maleiimide moiety.
 38. A compound according to claim 37 wherein X is an aromatic carbocyclic group.
 39. The compound according to claim 38 wherein X is phenyl.
 40. The compound according to claim 37 wherein R is the remainder of a peptide having a terminal amino group of the formula


41. The compound according to claim 37 wherein R is the remainder of a peptide having a terminal amino group of the formula


42. The compound according to claim 37 wherein R is the remainder of a peptide having a terminal amino group of the formula


43. The compound according to claim 37 wherein R is the remainder of a peptide having a terminal amino group of the formula


44. The compound according to claim 37 wherein R is the remainder of a peptide having a terminal amino group of the formula


45. The compound according to claim 37 wherein R is the remainder of a peptide having a terminal amino group of the formula


46. The compound according to claim 37 wherein R is the remainder of a peptide having a terminal amino group of the formula


47. The compound according to claim 37 wherein R is the remainder of a peptide having a terminal amino group of the formula


48. The compound according to claim 37 wherein R is the remainder of a peptide having a terminal amino group of the formula


49. A compound according to claim 37 wherein R₁ is the remainder of a protein characterized by the presence of a thiol group bound thereby to the 3-position of the maleiimide moiety.
 50. A compound according to claim 37 wherein the protein is a thiolated enzyme.
 51. The compound according to claim 50 wherein the thiolated enzyme is thiolated alkaline phosphatase.
 52. The compound according to claim 50 wherein the thiolated enzyme is β-D-galactosidase.
 53. The compound according to claim 50 wherein the thiolated enzyme is thiolated peroxidase.
 54. The compound according to claim 50 wherein the thiolated enzyme is thiolated glucose oxidase.
 55. A compound of the formula

wherein X is loweralkylene, an aromatic carbocyclic group or a saturated carbocyclic group and R is the remainder of a peptide having a terminal amino group and characterized by the presence of internal amino and/or hydroxyl groups; and R₁ is the remainder of a compound characterized by the presence of a thiol group bound thereby to the 3-position of the maleiimide moiety, substantially free of compounds, in which at least one of the internal amino and/or hydroxyl groups is bound to the carbon atom of the amide carbonyl group.
 56. A compound according to claim 55 wherein X is an aromatic carbocyclic group.
 57. The compound according to claim 56 wherein X is phenyl.
 58. The compound according to claim 55 wherein R is the remainder of a peptide having a terminal amino group of the formula


59. The compound according to claim 55 wherein R is the remainder of a peptide having a terminal amino group of the formula


60. The compound according to claim 55 wherein R is the remainder of a peptide having a terminal amino group of the formula


61. The compound according to claim 55 wherein R is the remainder of a peptide having a terminal amino group of the formula


62. The compound according to claim 55 wherein R is the remainder of a peptide having a terminal amino group of the formula


63. The compound according to claim 55 wherein R is the remainder of a peptide having a terminal amino group of the formula


64. The compound according to claim 55 wherein R is the remainder of a peptide having a terminal amino group of the formula


65. The compound according to claim 55 wherein R is the remainder of a peptide having a terminal amino group of the formula


66. The compound according to claim 55 wherein R is the remainder of a peptide having a terminal amino group of the formula


67. A compound according to claim 55 wherein R₁ is the remainder of a protein characterized by the presence of a thiol group bound thereby to the 3-position of the maleiimide moiety.
 68. A compound according to claim 55 wherein the protein is a thiolated enzyme.
 69. The compound according to claim 55 wherein the thiolated enzyme is thiolated alkaline phosphatase.
 70. The compound according to claim 68 wherein the thiolated enzyme is β-D-galactosidase.
 71. The compound according to claim 68 wherein the thiolated enzyme is thiolated peroxidase.
 72. The compound according to claim 68 wherein the thiolated enzyme is thiolated glucose oxidase.
 73. A solid support for use in immunoassays comprising a compound of the formula

wherein X is loweralkylene, an aromatic carbocyclic group or a saturated carbocyclic group and R is the remainder of a peptide having a terminal amino group and characterized by the presence of internal amino and/or hydroxyl groups, in which the terminal amino group is bound to the carbon atom of the amide carbonyl group, substantially free of compounds, in which at least one of the internal amino and/or hydroxy groups is bound to the carbon atom of the amide carbonyl group; and R¹ is the remainder of a compound characterized by the presence of a thiol group bound thereby to the 3-position of the maleiimide moiety.
 74. A method of determining an analyte in a sample of interest comprising the steps of: a. incubating a sample of interest with a compound of the formula

wherein X is loweralkylene, an aromatic carbocyclic group or a saturated carbocyclic group and R is the remainder of a peptide having a terminal amino group and characterized by the presence of internal amino and/or hydroxyl groups, substantially free of compounds, in which at least one of the internal amino and/or hydroxyl groups is bound to the carbon atom of the amide carbonyl group; and R₁ is the remainder of a compound characterized by the presence of a thiol group bound thereby to the 3-position of the maleiimide moiety to provide an analyte/conjugate mixture; b. washing the analyte/conjugate mixture with a substrate from an analyte/conjugate/substrate mixture; c. incubating the analyte/conjugate/substrate mixture; and d. measuring the amount of analyte.
 75. A method of claim 74 wherein X is an aromatic carbocyclic group.
 76. A method of claim 75 wherein X is phenyl.
 77. The method according to claim 76 wherein R is the remainder of a peptide having a terminal amino group of the formula


78. The method according to claim 74 wherein R is the remainder of a peptide having a terminal amino group of the formula


79. The method according to claim 74 wherein R is the remainder of a peptide having a terminal amino group of the formula


80. The method according to claim 74 wherein R is the remainder of a peptide having a terminal amino group of the formula


81. The method according to claim 74 wherein R is the remainder of a peptide having a terminal amino group of the formula


82. The method according to claim 74 wherein R is the remainder of a peptide having a terminal amino group of the formula


83. The method according to claim 74 wherein R is the remainder of a peptide having a terminal amino group of the formula


84. The method according to claim 74 wherein R is the remainder of a peptide having a terminal amino group of the formula


85. The method according to claim 74 wherein R is the remainder of a peptide having a terminal amino group of the formula


86. A method of therapeutic treatment of disease comprising administering to a mammal requiring therapeutic disease treatment, a therapeutic disease treating effective amount of a compound of claim
 1. 87. A method of therapeutic treatment of disease comprising administering to a mammal requiring therapeutic disease treatment, a therapeutic disease treating effective amount of a compound of claim
 13. 88. A compound according to claim 37 wherein R₁ is the remainder of a protein characterized by the presence of a third group bound thereby to the 3-position of the maleiimide moiety.
 89. A compound according to claim 55 wherein R₁ is the remainder of a protein characterized by the presence of a third group bound thereby to the 3-position of the maleiimide moiety.
 90. A solid support for use in immunoassays comprising a compound of the formula

wherein X is loweralkylene, an aromatic carbocyclic group or a saturated carbocyclic group and R is the remainder of a peptide having a terminal amino group and characterized by the presence of internal amino and/or hydroxyl groups, in which the terminal amino group is bound to the carbon atom of the amide carbonyl group, substantially free of compounds, in which at least one of the internal amino and/or hydroxy groups is bound to the carbon atom of the amide carbonyl group; and R¹ is the remainder of a compound characterized by the presence of a thiol group bound thereby to the 3-position of the maleiimide moiety.
 91. The solid support according to claim 90 wherein R₁ is a protein.
 92. The solid support according to claim 90 wherein R₁ is an enzyme.
 93. The solid support according to claim 90 wherein R₁ is an antibody. 